A significant portion of lymphocyte differentiation occurs in the bone marrow where lymphocytes first separate themselves from other hemopoietic cells and where T cells initially segregate themselves from B cells. Lymphocytes therefore make important maturational decisions in this compartment of differentiation. The heterogeneity of the bone marrow cell population has made dissection of this compartment difficult, and our approach has been to define cell surface antigens to identify different cell types and to find tumor analogs that could provide useful homogeneous clones of cells. We have data demonstrating that Abelson murine leukemia virus can be used profitably in two ways: 1. to mark hemopoietic and lymphoid progenitor cells; 2. to provide cloned transformed lines of such progenitors. We propose to extend these studies with new attention to the in vitro induction of the cell lines to express more differentiated surface phenotypes.